In a prior art apparatus of the kind specified above for forming a slit in a rock and a concrete surface, such as the one disclosed in Japanese Laid-Open Patent Application No. SHO 60-5996, an apparatus for making a plurality of holes uses a plurality of drills. Further, a method of continuously making holes in partially overlapped relationship is disclosed in Japanese Laid-Open Patent Application No. SHO 61-31591.
While it has been expected to develop an apparatus for forming a slit efficiently in a rock and a concrete surface, the above-mentioned prior apparatus and method have posed the following problems.
(1) A low drilling speed. PA1 (2) A complicated construction. PA1 (3) Because interconnected slits are formed, irregularities remain in the inner surfaces of the slits.